The 3GPP (3rd-Generation Partnership Project) has specified broadcast and multicast services for GSM (Global System for Mobile Communications) and UMTS (Universal Mobile Telecommunications System) networks, namely the Multimedia Broadcast and Multicast Service (MBMS) feature. With MBMS, streaming services may be provided. As an example for a streaming service, a mobile TV service may be mentioned, which can be characterized in that content is continuously provided and that a user (sub-scriber) may be interested to join the service at any time.
The MBMS feature is specified, e.g., in the 3GPP TS (Technical Specification) 23.246, TS 25.346 and TS 26.346. MBMS adds a plurality of broadcast/multicast-related techniques to conventional GSM or UMTS networks for the distribution of content such as video, audio or in general multimedia data. Each broadcast/multicast service is related to a particular service area—the geographical area or zone, within which the content can be broadcasted or transmitted.
A so-called broadcast/multicast service centre (BM-SC) is defined in the Technical Specifications as dedicated MBMS control node. Within the MBMS framework, the BM-SC is responsible for providing and delivering broadcast/multicast services. The BM-SC serves as an entry point for content-delivery services, sets up and controls MBMS transport bearers, and may additionally be used to initiate MBMS content transmissions.
Downstream of the BM-SC, the MBMS content is forwarded along a hierarchically organized content distribution tree, wherein the radio access nodes serving a single radio cell form the leaves of the tree. In an UMTS access network, the Radio Network Controllers (RNCs) or Node Bs (or ‘evolved Node Bs’ in an LTE network) act as further control nodes for the content distribution. For example, these nodes determine whether to set up a PTM bearer, a point-to-point (PTP) bearer, or no radio bearer at all for the content distribution in response to some feedback received from user terminals.
In MBMS, a control channel termed multimedia control channel (MCCH) is provided for broadcasting control information into the service area. The control information comprises critical and non-critical information. Generally speaking, critical information informs the potential recipients of the content distribution on, for example, when and how to receive a content distribution provided on an MBMS traffic channel (MTCH). Thus the critical information comprises service information and radio bearer information (i.e., for example, a service identifier, multicast addresses and time of transmission, but also media descriptions).
Non-critical information provided in the MCCH may include counting request information. A counting request asks the recipients to indicate to the network if they are interested in a reception of a content distribution. The counting procedure allows the RNC/Node B to decide upon the type of radio transmission bearer to be allocated (PTM bearer, PTP bearer or no bearer).
The MCCH information is transmitted based on a particular schedule related to the actual content distribution. Typically, control information about forthcoming content distributions is continuously provided in the MCCH during a time span extending from a certain point in time before the content distribution until the end of the distribution (more precisely, the end of the last session). This allows a user to join a particular service at any time. For example, control information is provided on the MCCH beginning from a Service Announcement provided by the BM-SC until after the stop of the last session (cf. TS 23.246, FIG. 5).
So-called file delivery services are similar to streaming services in some respect. An example for a file delivery (or ‘file download’) service is a Podcast service. A Podcast service typically comprises three steps. In a first step, the user is required to subscribe to the Podcast service (this may also be a mandatory step for any other service). For example, the user might subscribe to a TV series, an audio book, a radio program, a compilation of music songs etc. In a second step, data items (i.e. files) are delivered to the user terminal of the user, whereby typically each file is delivered independently from the others. For example, the user may have subscribed for a weekly TV show. Then, each week a content file representing the particular TV show of this week is downloaded (pushed) to the user terminal as soon as the file is available. A preferred delivery time (download time) may be overnight. The delivered content file is stored in the user terminal. Eventually, in the third step the user may consume the stored content file at any time he or she prefers. Thus, while a file download service such as a Podcast service is similar to a streaming service in that one and the same content is to be distributed to many users (subscribers), there is a difference in the intended usage of the content: While a TV subscriber may join a service at any time and consumes the streaming content essentially immediately, a Podcast subscriber is interested in obtaining the content file to be provided for download in its entirety but will consume it at a later time.
Irrespective of whether the content distribution comprises a streaming service or a file delivery service, in MBMS the MCCH resources for announcing the content distribution and for providing related control information are allocated before and during the entire content distribution. The MCCH has a limited transmission capacity, such that only a limited number of services can simultaneously be announced. In a typical MBMS-enabled network, 20 to 30 MBMS bearers can be announced in parallel.